A. Definitions
As used in this specification and in the claims, the following terms shall have the following meanings:
(1) Optical Component
An optical component is a component which has optical power and/or corrects one or more monochromatic and/or one or more chromatic aberrations and which requires separate mounting and alignment from other components of the projection lens.
As illustrated by the examples present below, optical components include, for example, single lens elements and cemented doublets. Projection lenses having less optical components are preferred to projection lenses having more optical components because having less components simplifies assembly and generally results in a reduction in a projection lens' weight and component cost.
(2) Barrel Length
Barrel length (BRL) is the distance between the vertex of the front surface of the forward-most optical component of the projection lens and the vertex of the back surface of the rearward-most optical component.
(3) Off-Axis Power of an Aspherical Lens Element
The power of an aspherical lens element at an off-axis position, e.g., at a position y equal to 0.7 times the clear aperture of the lens element's short conjugate surface, is given by:φy=(n−1)(C1y−C2y)where n is the index of refraction of the lens element (specifically, the index of refraction at 546.1 nanometers), and C1β and C2β are, respectively, the local curvatures of the long conjugate and short conjugate surfaces of the lens element at the height y, which, in accordance with conventional practice, are positive when the center of curvature is on the right, e.g., short conjugate, side of the surface.
(4) Best Fit Spherical Surface to an Aspherical Surface
For lens surfaces that are aspheric, in addition to the radius of curvature at the optical axis, the overall shape of the surface and thus of the lens element which comprises the surface can be described in terms of best fit spherical surfaces. As discussed below, in this specification and in the claims, best fit spherical surfaces are used to describe the shape of the L2 lens element.
Procedures for determining best fit spherical surfaces can be found in Dunham, C. B., and C. R. Crawford, “Minimax Approximation by a Semi-Circle,” Society for Industrial and Applied Mathematics, Vol. 17, No. 1, February, 1980, pages 63–65. In many cases, the description of a lens element, e.g., as being biconcave, will be the same whether the surfaces of the element are described in terms of best fit spherical surfaces or in terms of the radii of curvature at the optical axis.
(5) Abbe Number
Abbe numbers are calculated using the formula:v=(n546.1−1)/(n480.0−n643.8)where n480.0, n546.1, and n643.8 are the indices of refraction of the optical material at 480.0, 546.1, and 643.8 nanometers, respectively.
B. Projection Systems
Image projection systems are used to form an image of an object, such as a display panel, on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
FIG. 14 shows in simplified form the basic components of an image projection system 17 for use with a pixelized imaging device (also known in the art as a “digital light valve”). In this figure, 10 is an illumination system, which comprises a light source 11 and illumination optics 12 which transfer some of the light from the light source towards the screen, 13 is the imaging device, and 14 is a projection lens which forms an enlarged image of the imaging device on viewing screen 15. For front projection systems, the viewer will be on the left side of screen 15 in FIG. 14, while for rear projection systems, the viewer will be on the right side of the screen.
For ease of presentation, FIG. 14 shows the components of the system in a linear arrangement. For a transmissive LCD imaging device and, in particular, for a rear projection system employing a large format transmissive LCD imaging device of the type with which the present invention will typically be used, the optical path between the imaging device and the screen preferably includes two folds so as to reduce the overall size of the cabinet used to house the system. In particular, a first fold mirror is preferably placed between imaging device 13 and projection lens 14 and a second fold mirror is preferably placed between the projection lens and screen 15.
The linear arrangement shown in FIG. 14 is also modified in the case of a reflective imaging device. Specifically, in this case, the illumination system is arranged so that light from that system reflects off of the imaging device, i.e., the light impinges on the front of the imaging device as opposed to the back of the device as shown in FIG. 14. Also, for such imaging devices, one or more prism assemblies will be located in front of the imaging device and will receive illumination light from the illumination system and will provide imaging light to the projection lens.
Image projection systems preferably employ a single projection lens which forms an image of: (1) a single imaging device which produces, either sequentially or simultaneously, the red, green, and blue components of the final image; or (2) three imaging devices, one for red light, a second for green light, and a third for blue light. Rather than using one or three imaging devices, some image projection systems have used two or up to six imagers. Also, for certain applications, e.g., large image rear projection systems, multiple projection lenses are used, with each lens and its associated imaging device(s) producing a portion of the overall image. In the preferred embodiments of the present invention, a single projection lens is used to form an image of a single imaging device, e.g., a large format transmissive LCD panel.
A particularly important application of projection systems employing pixelized panels is in the area of rear projection systems which can be used as large screen projection televisions (PTVs) and/or computer monitors. To compete effectively with the established cathode ray tube (CRT) technology, projection systems based on pixelized panels need to be smaller in size and lower in weight than CRT systems having the same screen size.
C. Optical Performance
To display images having a high information content (e.g., to display data), a projection lens needs to have a high level of aberration correction. In particular, the lens needs to have a high level of resolution across the entire field of the lens and a high-level of chromatic aberration correction.
A high level of resolution is needed because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, a clear, undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing CRTs a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction is thus needed from the projection lens.
High resolution and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of resolution loss or color aberration in the images of such interfaces.
D. Cabinet Size
For rear projection systems, there is an ever increasing demand for smaller cabinet sizes (smaller footprints). In terms of the projection lens, this translates into a requirement that the lens has a wide field of view (FOV) in the direction of the image (screen). The requirement for a large FOV makes it even more difficult to correct the lateral color of the lens.
In addition to a large FOV in the direction of the lens' long conjugate, when used with a large format pixelized panel, the projection lens also needs to have a relatively large FOV in the direction of its short conjugate. However, such short conjugate FOV must not be so large as to compromise the transmission of light through the Fresnel lens typically used on the projection lens side of a transmissive LCD panel.
Achieving wide fields of view in the direction of the lens' long and short conjugates, while still maintaining high levels of aberration correction, is technically challenging. To do so while minimizing the size of the projection lens and the number of optical components used in the lens is even more demanding. As illustrated by the examples presented below, the present invention in its preferred embodiments provides projection lenses which simultaneously satisfy these competing design criteria.